Telephone ringing apparatus



Nov. 30, 1943. R. H. HERRICK 2,335,512

TELEPHONE RINGING APPARATUS Filed July 3, 1941 3 Sheets-Sheet 1 llb 'LINE SWITCH SELECTOR TO comEc'roFzs INVENTOR. ROSWELL H. HERRICKATTORNEYS NOV. 30, 1943. R HERRlCK 2,335,512

TELEPHONE RINGI'NG APPARATUS Filed July 3 1941 3 Sheets-Sheet 2 MARGINALCONVERTING UNIT l7 RELAYS l9 iilBRATOR NO. 2

I VIBRATOR No.3 T?

RA OR N0 INVENTOR. ROSWELL H. HERRIOK M02547, M M

ATTORNEYS Nov. 30, 1943. R. H. HERRICK TELEPHONE RING-ING' APPARATUSFiled July}, 1941 5 Sheets-Sheet 3 lllll INVENTOR. ROSWELL H. HERRICK unthy-2 358 I'll Ill-Ill ATTORNEYS Patented Nov. 30, 1943 TELEPHONERENGING APPARATUS Roswell H. Herrick, Oak Park, 111., assignor toAutomatic Electric Laboratories, Inc., a corporation of DelawareApplication July 3, 1941, gerial No. 426M335 24 Claims.

The present invention relates to telephone ringing apparatus and moreparticularly to improvements in ringing apparatus of the character usedin small exchanges where facilities for selective ringing on party linesare required.

In installations of this character, ringing currents of the requiredfrequency or frequencies are derived from the exchange battery throughconverter channels which usually include converters of the pole changertype. This type of converter conventionally includes a pole changerrelay which functions alternately to energize the two halves of theprimary winding of an associated transformer from the exchange battery,so that an alternating voltage of the desired frequency is producedacross the secondary winding of the transformer. Under conditions ofheavy load, when the contacts of the pole changer relay are required tomake and break the highly reactive circuits over which the transformerprimary windings are alternately energized, serious and objectionablearcing and pitting of the contacts may occur.

It is an object of the present invention, therefore, to provide improvedringing apparatus of the character described wherein no moving parts areused in converting the direct voltage of the available direct currentsource into alternating current of the desired frequency.

It is another object of the invention to provide improved ringingapparatus of the pole changer type wherein provisions are made forlimiting the currents traversing the contacts of a pole changer relay toexceedingly minute values regardless of the load demands on theapparatus.

. According to another and more specific object of the invention, thevariations in the currents traversing the primary windings of theconversion transformer are effected through appropriate control ofthermionic space current paths which are included in circuit with theprimary windings.

In accordance with another object of the invention provisions are madefor automatically establishing an alternative conversion channel, fromwhich the space current paths of the thermionic means are excluded, whencertain abnormal conditions, such, for example, as failure of thethermionic means, are present in the apparatus.

According to another object of the invention improved and exceedinglysimple arrangement is provided for indicating the condition of theapparatus and for producing a distinctive signal indication when anabnormal condition is present therein.

It is another object of the invention to provide improved ringingapparatus of the character described which operates to provide aplurality of output frequencies in a positive, reliable and improvedmanner and includes a minimum of circuit equipment.

According to another and more specific object of the invention a singletransformer is utilized in the apparatus, successively to deliver outputcurrents of the required difierent frequencies.

The novel features believed to be characteristic of the invention areset forth with particularity in the appended claims. The invention, bothas to its organization and method of operation, together with furtherobjects and advantages thereof, will best be understood by reference tothe specification taken in connection with the accompanying drawings, inwhich Figs. 1 and 2, when laid one above the other in the order named,illustrate an automatic telephone system having incorporated thereinimproved ringing apparatus constructed and arranged in accordance withthe present invention; and Fig. 3 illustrates a modified arrangement ofthe ringing apparatus which may be used in conjunction with theautomatic switching equipment shown in Fig. 1 in lieu of the ringingapparatus shown in Fig. 2.

Referring now more particularly to Figs. 1 and 2 of the drawings, thesystem there illustrated may be provided in a small unattended exchangewhich serves one thousand lines or less. The system includes a pluralityof single party lines,

- one of which is indicated at H] as extending to the substation A, anda plurality of multiparty lines one of which is indicated at H asserving the substations B and C. These lines terminate in the exchangein line switches 20 and 2|, respectively. The automatic switchingequipment of the exchange further includes a plurality of selectors, oneof which is indicated at 25, which are accessible to the various lineswitches and in turn have access to a plurality of groups of connectors,one of which includes the connector 30. Preferably the line switches 20,2|, etc., are of the well-known rotary type of which several commercialembodiments are available. Each of the selector and connector switchesof the system is preferably of the well-known Strowger type havingembodied therein a wiper carriage structure which is adapted to betranslated vertically to bring the wipers thereof to a position oppositea particular level of bank contacts and then rotated to bring the wipersinto engagement with a particular set of contacts in the selected level.Such movement of the Strowger switch wipers is conventionally obtainedby providing vertical and rotary magnets and suitable relay equipmentfor controlling these magnets and for effecting the required signalingand line switching operations. If desired, the selector 25 may be of theform illustrated and described at page 69 at seq. of the book entitledTelephone Theory and PracticeAutomatic Switching and AuxiliaryEquipment, Written by Kempster B. Miller and published in 1933. Theconnector 30 may be of the character disclosed at pages 82, 83 and 84 ofthis book.

In order more clearly to explain the operation of the circuits to whichthe invention pertains, a

portion of the control equipment embodied in the connector 30 has beenillustrated in detail. More specifically this equipment comprises aslow-tooperate ringing control relay RIIIO, a slow-acting ring cut-offrelay RI I and a ringing frequency selecting switch I20 of thewell-known minor type. In brief the switch I20 comprises a single set ofbank contacts I2I, a wiper I22, an operating magnet I23 for driving thewiper I22 over the contacts of the contact set I 2 I, a release magnetI24 which when operated permits the wiper I22 to be restored to normal,and a set of oil normal springs I25. As indicated above the lineswitches individual to the various subscriber lines are of thewell-known rotary type. The switch 2| is illustrated as including a linerelay RIM) which includes windings normally bridged across theconductors of the line II. If desired, these switches may be of the formdisclosed at pages 48, 49, 50 and 51 of the above-cited book written byMiller.

The substation equipment provided at each of the substations A, B, C,etc., of the system may be of any desired commercial form. In brief,each of these substations includes a transmitter, a receiver, a bell orother form of signal device, an anti-side-tone induction coil and a hookor cradle switch which is arranged to be controlled by the associatedreceiver to complete a substation circuit of the well-knownanti-sidetone type. In this regard it is pointed out that the ringersprovided at the substations served by the multiparty lines of the systemare of the well-known tuned reed type, such that each thereof Willrespond only to ringing current of a particular frequency.

The ringing apparatus which. is provided in the exchange is arranged tobe controlled by the automatic switching equipment just described andfunctions to provide the difierent ringing current frequencies throughwhich selective signaling over the multiparty lines is eflected. Moreparticularly, the operation of the equipment is controlled by a manuallyoperable switch 236 and a start relay R235 which is arranged to beenergized over the common start lead 295. Briefly considered, thisequipment comprises a pair of converting units 51 and I3 which mayselectively be used in converting the direct cur-- rent of the exchangebatter I5 into alternating current of the desired frequency. Morespecifically, a transfer relay R289 is provided for selectivelyassociating the two converting units I1 and I8 with the pulse generatorsSI, 32, 33, etc., having different pulse periodicities. For the purposeof successively connecting the different pulse generators, one at atime, to the one of two converting units selected by the transfer relayR280, a set of cyclically operating sequence relays I9 is provided. Thisset of relays includes a timing relay RZ IB, transfer relays R250, R250,R2Ic, etc., individual to th several pulse generators 3I, 32, 33, etc.,and a reset relay R290. Each of the several pulse generators 3H, 32, 33,etc., is of the pole changer type and includes a relay provided with aweighted armature having a specific period of vibration. For example,the pulse generator SI comprise a relay 225 which is equipped with aWeighted armature 226 having a natural period of vibration of 16 cyclesper second. The relay comprising the generator 32 may be equipped with aweighted armature designed to vibrate at 33 cycles per second and therelay comprising the generator 33 may be designed to vibrate at a rateof 50 cycles per second.

More specifically considered, the converting unit I'I comprises atransformer 29!! having a pair of primary windings or winding section2022 and 283, and a secondary winding 25H which is arranged successivelyto be connected to the ringing conductors 292, 293, 294, etc., duringeach cycle of operation of the sequence relays I9. The secondary winding2M is shunted by a condenser Edla having the function of providing a lowimpedance path for the radio frequency components of the alternatingcurrent developed during operation of the unit. The two primary windings202 and 203 are arranged to be energized in opposite directions from asource of di rect current 35 over circuits which respectively includethe space current paths of two electron discharge tubes 284 and 285.Each of these tubes is of the well-known pentode type utilizing anindirectly heated cathode. For example, the tube 264 comprises an anode204a and a cathode 2940 separated to define a space current path, and acontrol electrode 2841) which may be subjected to a fixed or varyingpotential to determine the current flow over the space current path. Thetube further comprises a suppressor grid which is directly connected tothe cathode of the tube, a screen electrode 204d, and a cathode heater204e, For the purpose of normally biasing the control electrodes 2342iand 2051) of the two tubes 2M and 2215 to substantially the samenegative potentials so that currents of substantially equal magnitudenormally traverse the two primary windings 2e: and 263, a biasingnetwork i provided which includes a voltage dividing resistor 20'!bridged across the exchange battery I5. It will be noted that thevoltage across the resistor section 28111 is negatively applied to thecathodes 2840 and 2050 of the two tubes and that the larger voltageappearing across the two series connected sections 251a and Zil'lb inegatively applied to the control electrodes 2341) and 2552) through theresistors 208 and 2% respectively. In order to vary the potential ofthese control electrodes in accordance with the operation of any one ofthe pulse generators to which the converting unit I 1 may be connected,circuits are provided whereby these electrodes may be controlled throughthe resistors 2H) and ZII respectively, the contacts of the sequencerelays I9, and the contacts of the pulse generators 3!, 32, 33, etc.

In general, the arrangement of the converting unit I8 is similar to thatof the converting unit IT as described above. Thus the unit I3 comprisesa transformer 2I3 which is provided with a secondary winding 2M having aresistor 2I8 and a condenser 2 I9 connected in shunt therewith andarranged to be successively connected to the ringing current leads 2,92,293, 294, etc., by the sequence relays l9 when the transfer relay R289is restored. The transformer 213 .further comprises a pair of primarywindings H and H5 which are bridged by the condenser 2H and are arrangedto be alternately energized in opposite directions from the exchangebattery i5 under the control of the pulse generators 3i, etc. Theenergizing circuits for these two windings commonly include. alow-resistance, high impedance, iron-core choke 22] which is providedfor the purpose of suppressing noise currents that may be producedduring operation of the unit. Further to this end, a condenser 223 isprovided which shunts each of the two windings 2G5 and 216 during thoseperiods when the windings) are respectively energized.

As indicated by the above explanation, when the converting unit I8 isconnected for use, the primary windings 2E5 and M6 included therein arearranged for .direct energization under the control of the pulsegenerators 3!, 32, 3-3, etc, so that conditions are established wherebythe pole changing or circuit controlling contacts of the pole changingrelays may be subjected to arcing and sparking when heavy load demandsare imposed on the system. In order to minimize these effects thetransformer 22.3 is designed to possess a high leakage reactance and acondenser 2!? having a compromise capacitance value, is used. In thisregard it may be pointed out that if a condenser 2!? of largecapacitance is used, sparking at the pole changing contacts during thecircuit break periods may be substantially entirely eliminated. However,the energy storage in a condenser of this size results in an enhancedpitting of the contacts during the make periods thereof. On the otherhand if a small condenser is used the pitting may be substantiallyentirely eliminated but the spark suppression obtained is totallyinadequate. By effecting a compromise between these two values, however,reasonably good spark suppression, accompanied by a not objectionableamount of pitting, may be obtained regardless of the load demands on thesystem.

As indicated above, the transfer relay R5238 is provided for selectivelyassociating the two converting units l! and ill with the remainingportions of the ringing equipment. It is more specifically pointed outthat the converting unit H is normally utilized in the production of thealternati. currents of different frequencies. To this end, a marginalrelay R295 is provided which is included in the common portion of, thepaths over which the positive terminal of the current source 35 isconnected to the two screen electrodes 284i) and Z liib. This relay ismargihed to operate and hold its operated positio when traversed bycurrent of a value approximatel one and one-half times the screenelectrode current of each tube. As thus margined the relay R285 normallyholds its operated position, but will r store if the screen current flowto either of the two tube .is interrupted. In order to exclude thewinding of the relay R295 and the current source 35 from the pathstraversed by the periodically changing output currents of the two tubes2M and 285, a pair of by-p-ass condensers 23'? and 20.6 are providedwhich respectively shunt the relay R235 and the source 35.

Neglecting for the present the mode of operation of the ringingapparatus illustrated in 2 of the drawings and the manner in which thisapparatus is controlled, the system as described above is substantiallyconventional in form and its operation is well understood in the art. Inbrief when a subscriber such, for example, as the subscriber at thesubstation A lifts the receiver provided at this substation from itssupporting hook or cradle to initiate a call intended for the substationB, for example, a loop circuit is completed for initiating the operationof the line'switch 25] to select a trunk leading to an idle selector. Ifthe selector 25 is seized by the line switch 251, the calling loopcircuit is extended to this selector and the selector is conditioned torespond to the first series of impulses dialed at the calling substationA. With the selector 25 in this condition, the usual dial tone signal isreturned over the established loop circuit to the calling subscriber toindicate that the dialing operation may be started. When the first digitis dialed at the calling substation, the dial tone signal is arrestedand the wipers of the selector 25 are elevated to a position oppositethe level of bank contacts terminating the trunks leading to theconnectors having access to the desired line i 5. During the interdigitpause between the first and second digits, the wipers of the selector 25are automatically stepped over the contacts of the selected level untila trunk leading to an idle connector of the selected group is found.Assuming that the connector 353 is the first available idle connector ofthe group, the calling loop circuit is extended to this connector whenthe wipers of the selector 25 are stepped into engagement with thecontacts terminating the trunk line extending to this connector. Whenthe connector M is thus seized, certain of the control relays embodiedtherein and more particularly the line and release relays, operate tocomplete a path for impressing ground potential upon the start conductor2295 thereby to initiate the operation of the rin ing apparatus in themanner explained below. In response to the dialing of the second digitthe wipers 0f the connector 31! are stepped vertically to a positionopposite the level of bank contacts in which are terminated the lines ofthe ten line subgroup including the called line it. Following this digitand during the interdigit pause between the second and third digits, theconnector is conditioned to rotate its wipers step by step in accordancewith the impulses of the third digit of the directory number designatingthe line i 5. When the third digit is dialed, the wipers of theconnector Si! are positioned to engage the particular contactsterminating this line and at the end of the digit the control circuitsof the connector are conditioned to repeat the impulses of the fourthdigit to the operating magnet I23 of the frequency selecting switch 526.This magnet responds to the impulses of the fourth digit by operatingthe wiper I22 into engagement with the particular contact of the contactset l2! having impressed thereon ringing voltage of the particularfrequency required to operate the harmonic ringer provided at thedesired substation B. If it be assumed, for example, that the ringerprovided at this substation is constructed to respond only to ringingcurrent of 33 cycles the fourth digit dialed at the calling substationwill comprise two impulses so that at the conclusion thereof the wiperI22 is left standing in engagement with the contact terminating theringing conductor 2%. During the operation of the switch i253 and whenthe wiper l2? is stepped oil normal, the oil-normal springs 1525 areclosed to prepare the operating circuit forthe release magnet I24, thiscircuit being held open through break contacts controlled by the releaserelay of the connector until the connection is released.

In accordance with conventional practice suitable provisions are made inthe connector 30 for testing th selected line II to determine the idleor busy condition thereof, for returning the usual busy tone signal overthe established loop circuit to the calling subscriber in the event thecalled line is busy, and for projecting ringing current of the selectedfrequency over the called line in the event this line is idle. Assumingthat the called line is idle, a circuit is completed through operationof certain of the relays embodied in the connector 39 for energizing thelower winding of the slow-to-operate ringing control relay RI Oil inseries with the winding of the cut-off relay conventionally embodied inthe line switch 2!. When this circuit is closed the cut-off relay of theline switch 2I partially operates to disconnect the windings of the linerelay RI3II from the conductors of the called line II, thereby to clearthis line of the shunt impedance represented by these windings. Therelay RIIlIl also operates when energized in this series circuit. Inoperating, this relay first locks to ground over a prepared path whichincludes its preliminary make contacts I82. At its oontacts'llil andI03, the relay RIDU completes a circuit for projecting ringing currentof the selected frequency over the conductors of the line I I. In thepresent case the circuit traversed by the ringing current may be tracedas extending from the upper terminal of one of the two transformerwindings 2IlI or 2H, the contacts 28! or 282, the contacts 2', MI, etc.,the ringing conductor 293, the wiper I22 and its engaged second contact,the contacts H5 and I03; the conductor I ID, the windings of the ringerprovided at the substation B, the conductor No, the contacts IIII andIII, the upper winding of RI I I] and the exchange battery I5 to theopposite or grounded side of the transformer winding 20I or thetransformer winding 2I4. From a consideration of this circuit it will benoted that the illustrated X wiring is used throughout in the ringingapparatus illustrated in Fig. 2 of the drawings. With this wiringarrangement, grounded ringing is utilized in signaling over all thelines of the system. If, however, the requirements of the system aresuch that superimposed battery ringing must be used, the requirementsare met by utilizing the illustrated Y wiring arrangement in lieu of theillustrated X wiring arrangement.

Portions of the current transmitted over the above-traced ringingcircuit are obviously shunted through the harmonic ringers respectivelyprovided at the other substation served by the line II. The ringerprovided at the substation B is, however, the only ringer associatedwith this line which responds to current of the particular frequency of33 cycles. Operation of this ringer serves to signal the calledsubscriber that he is being called. A portion of the ringing current isalso shunted over the calling end of the connection, whereby thereceiver provided at the calling substation is energized to indicate tothe calling subscriber that the called substation is being signaled.

The desired talking circuit between the calling and called substationsis fully completed when the call is answered at the called substation B.More particularly, when the receiver provided at this substation isremoved from its supporting hook or cradle a direct current conductivebridge is connected across the conductors I la and I w of the line II tocomplete a circuit for energizing;

the upper winding of th slow-acting ring cutoff relay RI I 0. Thiscircuit extends from ground by way of the transformer winding 20I or 2I4in use, the contacts 282 or 28I, the contacts 21 I, 26I, etc., theringing conductor 293, the wiper I22, the contacts H5 and I03, theconductor II b, the bridge across the conductors IIb and Ila at thesubstation B, the conductor I Ia, the contacts IN and III, and the upperwinding of RI III to the negative terminal of the exchange battery I5.When its upper winding is energized in this circuit the relay RI IIIfirst locks to ground over a prepared path which includes thepreliminary make contacts II3. Thereafter the relay RIIO opens itscontacts III and H5 to interrupt its operating circuit as traced aboveand to open two points in the circuit for transmitting ringing currentover the line II. At the contacts I I2 and I it, the relay RI I0completes the desired talking circuit between the calling and calledsubstations. When this relay operates it also functions to open thepath, not shown, over which a portion of the ringing current istransmitted over the calling end of the connection, whereby the ringbacksignal is terminated in the usual manner.

Preferably the control relays of the connector 3! are so arranged thatthe release of the operated switch train is entirely under the controlof the calling subscriber at the substation A. With such an arrangement,the connection is cleared out at the calling substation when the loopcircuit extending through to the connector 30 is opened to cause therelease of the line and hold relays embodied in this switch. When theserelays restore, the line and selector switches 20 and 25 are releasedand the control apparatus embodied in the connector 30 is restored tonormal, all in a manner well understood in the art. During the releaseof the control apparatus in the connector 30 the previously tracedholding circuit for the two relays RIIlfl and RIIO are interrupted,causing these relays to restore. Also, the start conductor 295' isdisconnected from ground in the connector 30 and ground potential isremoved from the private conductor of the line II to cause thedeenergization and release of the cut-01f relay embodied in the lineswitch 2|. When the line and hold relay of the connector 30 restore acircuit is also completed for energizing the release magnet of theStrowger switching mechanism embodied in the connector 30, whereby thewiper carriage structure of the mechanism is restored to rotary andvertical normal. larly a circuit is completed by way of the offnormalsprings I25 for energizing the release magnet I24 of the frequencyselecting switch I28, whereby the wiper I22 of this switch is restoredto normal. Incident to the release of this switch, the off-normalsprings I25 are opened to deenergize the release magnet I24.

Referring more particularly to the operation of the ringing apparatusillustrated in Fig. 2 of the drawings, it is pointed out that tocondition this apparatus for operation the manually operable switch 236is operated to complete a circuit for energizing in multiple the cathodeheaters 2046 and 2056 of the two tubes 204 and 2%, respectively. Thiscircuit extends from the negative terminal of the exchange battery byway of the conductor I6, the contacts of the switch 236, the currentlimiting resistor 2I2, and the parallel-connected heaters 2Me and 2Il5eto the grounded positive terminal of the battery I5.

With these two heaters energized, the space current paths through thetwo tubes are rendered conductive to the extent permitted by thenegative biasing potentials impressed upon the control electrodes 204Dand 2051) from the voltage dividing resistor 201. Accordingly, theprimary windings 202 and 203 of the transformer 200 are energized inparallel circuits which commonly include the current source 35 andrespectively include the space current paths of the tubes 204 and 205.The relay R295 is also energized by the current, supplied by the source35, which traverses the screen electrodes 204d and 205d. With this relayoperated a circuit is completed at the contacts 296 for holding thetransfer relay R280 energized. With the relay R280 in its operatedposition, the contacts 283 and 285 are disengaged so that the convertingunit !8 is disassociated from the sequence relays l9 and the pulsegenerators 3!, 32, 33, etc. At the contacts 284 and 286 of the relayR280 paths are prepared over which control pulses of ground potentialmay alternately be impressed upon the two control electrodes 20417 and20512 through operation of one of the pulse generators 3!, 32, 33, etc.At the contacts 28! the circuit is held open over which the ringingconductors 252, 293, 294, etc., may be connected to the high potentialterminal of the secondary winding 2M. At the contacts 282 a circuit isprepared over which the ringing conductors may successively be connectedto the secondary winding 20! under the control of the sequence relays!9.

With the apparatus in the condition described above, if ground potentialis impressed upon the start conductor 295 through operation of one ofthe associated connectors in the manner previously described, an obviouscircuit is completed for energizing the start relay R255. This relay, inoperating, closes its contacts 25'! to complete a circuit for energizingin parallel the windings of the pole changer relays respectivelycomprising the pulse generators 3!, 32, 33, etc. One branch of thiscircuit extends from ground by wayof the series-connected right windingsof the relay 225, the resistor 235, the contacts 2 3'! and the conductorit to the negative terminal of the battery l5. When this branch circuitis completed the weighted armature 226 is attracted toward the polefaces of the relay 225 until the contacts 22'! and 228 are respectivelyengaged. When the contacts 22! are moved into engagement a circuit iscompleted for energizing the differentially related left windings of therelay 225. This circuit extends from ground by way of the armature 226,the contacts 22?, the inductance coil 23!, the series-connected leftwindings of the relay 225, the contacts 237, and the conductor 6 to thenegative terminal of the battery !5. When the relay 225 is thusdifierentially energized the attractive force exerted on the armature226 is reduced substantially to zero with the result that this armatureis released and swings through its normal position to a position whereinthe contacts 229 are engaged. Incident to this movement of the armature226 the contacts 22'! are opened to deenergize the left windings of therelay 225, and the contacts 228 are disengaged. The deenergization ofthe left windings of the relay 225 is retarded somewhat due to thepresence of the condenser 232 in order to permit the armature 226 toswing to the extreme position required for engagement of the contacts229. When the current traversing these windings is reduced substantiallyto zero an-attra'ct'ive force is again exerted on the armature 226 sothat this armature is again actuated to its attracted position whereinthe contacts 22? and 228 are respectively engaged. During thelast-mentioned movement of the armature 225 the contacts 229 areobviously opened. It will he understood from the above explanation thatthe vibration of the weighted armature 226' alternately to close thecontacts 228 and 2253 continues so long as the start relay R255 ismaintained in its operated position; It will also be understood that thecondenser 232 eiiectively shunts the contacts 22'! during the breakp'eriods'of these contacts for the purpose of minimizing sparkingthereat. Further, the action of the inductance coil 23! in the circuitcontrclied by the contacts 22? serves to prevent the generation oftransient currents of radio frequency during the break periods of thesecontacts. y

In operating, the relay R235 also closes its contacts 233 to complete acircuit for initiating the operation of the sequence relays I91 Morespecifically, when the contacts 235 are closed, a circuit including thecontacts 243' is completed for charging the condenser 246-fro1n theexchange battery i5. This condenser initially provides a low impedancepath in'shunt with the winding of the relay R250 for by-passing currentaround this relay winding, and thus functions to delay the operation ofthe relay R240. As a charge builds up across the electrodes of thiscondenser the shunting effect of the condenser decreases, so that aftera short interval the currenttraversing the winding of the relayRZMl andthe resistor 245 in series is sufiicient to cause the operation of thisrelay. In operating the relay R240 opens its contacts 243' to interruptits own operating circuit and to interrupt the charging circuit for thecondenser 246. After these circuits are opened the condenser 2 30discharges through the resistor 245 and the winding of the relay R240 inseries, thereby to maintain this relay in its operated position for agiven time interval. At its contacts 254 the relay R240 opens a point inthe operating circuit for the reset relay R290; At its contacts 25! therelay R240 opens the path normally short-circuiting thecondenser 249. Atits contacts 242 the relay R240 completes a circuit for charging thecondenser 249 in series with the winding of the sequence relay R250,this circuit extending from the grounded terminal of the exchangebattery I5'by way of the contacts 242, the condenser 249', the contacts255, the winding of R250, and the conductor It to the negative terminalof the battery !5; The initial rush of current over this circuit issufficient to cause the operation of the relay R2561 In operating thisrelay first locks to ground over a path including its preliminary makecontacts 256' and the contacts 29!. After this locking circuit iscompleted the relay R255 closes its contacts'25'4 to prepare a circuitfor charging the condenser 249 in series with the winding of the secondfrequency selecting relay R265 and opens its contacts 255 to interruptthe cir'cuit for charging the condenser 249 through its own winding. Bythe time the relay R250 has operated to perform the functions justdescribed" the condenser 249 is fully charged so that current flowthrough the winding of the relay R250isnrevehted.

After a given time intervalthe current which is passed throughvth'e'winding of the relay R240 from the condenser 24 6is' sufiicieritly"reduced to permit the release of thisrela'y. In restoring the relay R240opens its contacts 242 to interrupt the common portion of the availablecircuits over which the condenser 240 may be charged, and closes itscontacts 24! to discharge this condenser through the resistor 248. Atits contacts 244 the relay R240 reprepares the operating circuit for thereset relay R200. At its contacts 243 the relay R240 completes theabove-traced parallel circuits for charging the condenser 240 and forenergizing its own winding. It will be understood from the aboveexplanation that the relay R240 continues to operate and restore atintervals which are determined by the capacitance value of the condenser245 and the resistance values of the two resistors 245 and 241. Morespecifically, the resistance value of the resistor 245 determines therelease period of the relay R240, while the resistance value of theresistor 241 determines the period required to produce operation of thisrelay following its release. As the relay R240 continues to operate andrestore the sequence relays R200, R210, etc., which individuallycorrespond to the pulse generators 32, 33, etc. are successivelyenergized. Thus incident to the second operation of the relay R240, thecondenser 249 is charged in series with the winding of the relay R200over a circuit which extends from ground by way of the contacts 242, thecondenser 240, the contacts 254 and 251, the winding of R260 and theconductor !5 to the negative terminal of the battery Hi. When itswinding is thus energized the relay R200 first locks to ground over apath including its preliminary make contacts 208 and the contacts 20!.After this locking circuit is completed the relay R260 closes itscontacts 260 to extend the condenser charging circuit through thewinding of the next sequence relay and opens its contacts 261 tointerrupt the circuit over which the condenser 240 is energized inseries with its own Winding. The manner in which the other sequencerelays, indicated by dash lines in the drawings as being connectedbetween the relay R260 and the relay R210, are successively energizedand locked up in response to the continued intermittent operation of thetiming relay R240 will be clearly apparent from the precedingexplanation. After all of these intervening relays are operated thecharging circuit for the condenser 249 is extended to include thewinding of the last frequency selected relay R210. Accordingly when thetiming relay R240 next operates, the condenser 249 is charged in acircuit which includes the conductor 16, the winding of R210,

the contacts 216, the chain-connected contacts of the interveningfrequency selecting relays, the contacts 265 and 254, the con-denser 240and the contacts 242. When its winding is energized in this circuit therelay R210 locks to ground over a path including the contacts 211 and205. After this path is completed the relay R210 opens its contacts 210to interrupt the charging circuit for the condenser 249. Shortlyfollowing the operation of the relay R210 and when the timing relay R240next restores, the prepared operating circuit for the reset relay R2 90is completed, this circuit extending from ground by way of the contacts244 and 218, the resistor 203, the winding of R200, the resistor 204,and the conductor E0 to the negative terminal of the battery l5. It willbe noted that the Winding of the relay R200 is paralleled by thecondenser 202. Accordingly when the circuit just traced is completed themajor portion of the current is shunted through this condenser away fromthe winding of the relay R290 to delay the operation of this relay for ashort time interval. At the end of this interval the relay R290 operatesand opens its contacts 29l to interrupt the established lockingcircuitsfor each and all of the sequence relays R250, R250, R210, etc.Incident to the release of the relay R210 the contacts 210 are opened tointerrupt the circuit for energizing the relay R200. After this circuitis opened, the condenser 202 discharges through the resistor 294 and thewinding of the relay R290 to delay the release of this relay for a shorttime interval. At the end of this interval the relay R290 releases andcloses its contacts 29! to reprepare the locking circuits for thevarious sequence relays. Following the release of the sequence relaysand the reset relay R200, the network I9 is conditioned for a secondcycle of operation. In this regard it will be understood that the relaysof this network continue to operate in a cyclic manner so long as thestart relay R235 is maintained in its operated position.

Each time one of the sequence relays R250, R200, R210, etc., isoperated, the corresponding one of the pulse generators 3|, 32, 33,etc., is connected to control the potentials of the control electrodes2041) and 205?). Thus during each interval when the relay R250 isoperated the contacts 252 and 253 are closed to prepare paths over whichground potential may alternately be impressed upon these controlelectrodes by the relay 225 of the generator 3|. One of these pathsextends from ground by way of the armature 226, the contacts 229, 252,262, 212 and 204 and the resistor 2m to the control electrode 20422. Theother of the two paths extends from ground by way of the armature 226,the contacts 228, 253, 264, 214 and 286, and the resistor 21! to thecontrol electrode 2052). Each time the control electrode 2042) is thusconnected to ground the potential thereof is made positive with respectto the potential of the cathode 2040, so that the current traversing thespace current path of the tube 204 and the primary winding 202 in seriesis sharply increased. A correspondingly sharp decrease in the currenttraversing this circuit also occurs when the contacts 229 are open. Eachtime the contacts 228 ar closed to complete the path for impressingground potential upon the control electrode 2051) of the tube 205, thiselectrode is similarly made positive with respect to its associatedcathode 2050 so that the current traversing the series connected winding203 and space current path of the tube 205 is sharply increased. Acorresponding sharp decrease in this current occurs each time thecontacts 228 are opened to restore the normal negative bias to thecontrol electrode 20527. It will be understood that since the two pathsdescribed above are alternately completed under the control of the relay225 the current traversing the two primary windings 202 and 203 of thetransformer 200 is alternately increased and decreased, whereby analternatin induced voltage is developed in the secondary winding 20L Thefrequency of this voltage is obviously determined by the rate at whichthe two paths for impressing ground potential upon the controlelectrodes 204?) and 20517 are alternately completed and hence by therate of vibration of the weighted armature 226. This generatedalternating voltage has a more or less square wave form and accordinglyit is exceptionally well suited to the requirements of harmonic ringercontrol. Moreover the wave form of the generated alternating voltage iscompletely lacking in the high voltage peaks which normally occur in theoutput voltage of conventional pole changer converters at the beginningand end of each half cycle. Since the generated wave form is of thesquare top variety the harmonic content thereof is comparatively largebut is not so excessive as to cause cross ringing between the variouslines of the system.

After a predetermined time interval as measured by the operation of thetiming relay R240, the second sequence relay R260 is caused to operatein the manner explained above. In operating, this relay opens itscontacts 262 and 264 to disconnect the control electrodes 20412 and205?) from the contacts of the pole changer relay 225. At its contacts263 and 265 the relay R280 connects the control electrodes 2941) and 20%to the contacts of the pole changer relay comprising the pulse generator32. The manner, in which the contacts of the last mentioned relay arealternately closed to transmit ground pulses to the two controlelectrode 204b and 2051) is exactly the same as described above withreference to the operation of the relay 225. In the present case,however, the frequency at which these ground pulses are impressed uponthe two control electrodes is double the frequency at which the twoelectrodes are connected to ground by the relay 225. Accordingly, theoutput frequency of the apparatus is increased two-fold. In operating,the relay R260 also opens its contacts25l to disconnect the ringingconductor 232, which is individual to the generator 3I, the selectingrelay R250 and the first ringing frequency, from the high potentialterminal of the secondary winding 20!. At its contacts 26I' the relayR260 connectsthe second ringing conductor 293, which is individual tothe pulse generator 32, to the high potential terminal of the secondarywinding 20I. Thus during the period which separates the operation of therelay R260 from the operation of the next succeeding sequence relay,ringing voltage of the second ringing frequency is impressed upon theringing conductor 293. As the sequence relays continue to operate, thecontrol electrodes of the two tubes 204 and 205 are successivelyconnected to be controlled by different ones of the pulse generators,and the corresponding ringing conductors are successively connected tothe secondary winding 20 I. Thus when the last sequence relay R210operates, it opens its contacts 212 and 214 and closes its contacts 213and 215, whereby the potentials on the control electrodes 2041) and205?) are controlled in accordance with the operation of the pulsegenerator 33. In operating the relay R210 also opens its contacts 2' andcloses its contacts 21!, whereby the high potential terminal of thesecondary winding 20I is connected to the ringing conductor 294 and isdisconnected from the remaining ringing conductors.

The above described operation of the ringing apparatus continues untilthe start relay R235 is deenergized and restores. In releasing thisrelay opens its contacts 231 to interrupt the operating circuits for thepole changer relays respectively comprising the various pulse generators3|, 32, 33, etc. At its contacts 238 the relay R230 opens theabove-traced operating circuit for the timing, relay R240 whereby thecyclic operation of the sequence relays I9 is discontinued.

As indicated by the aboveexplanation the two relays R295 and R280 areheld energized so long as the converting unit I1 is operatingin itsnormal and intended manner. If, however, one of the two tubes 204 and205 should become defective or the current source 35 should fail torender the unit I1 inoperative, the current traversing the winding ofthe relay R295 is decreased to a value insuflicient to maintain thisrelay in its operated position. In releasing, the relay R295 opens itscontacts 296 to deenergize the relay R280. The latter relay, uponrestoring, opens its contacts 284 and 286 to interrupt the commonportion of the paths for transmitting ground pulses to the controlelectrodes of the tubes 204 and 205. At its contacts 283 and 235 therelay R280 connects the primary windings 2I5 and 2I6 of the transformer2I3 to be alternately energized in accordance with the operation of thepulse generators 3|, 32, 33, etc., during different periods of eachoperating cycle of the sequence relays I9. At its contacts 282 the relayR280 opens a point in the common portion of the paths over which thevarious ringing conductors 292, 293, 294, etc., may be connected to thehigh potential terminal of the secondary winding 20I. At its contacts28I the relay R230 prepares a common path over which the ringingconductors may successively be connected to the high potential terminalof the secondary winding 2I4 during each cycle of operation of thenetwork I9. Thus following the release of the transfer relay R280 theconverting unit I8 is included in the conversion channel in lieu of theconverting unit I1. In this regard it will be noted that with the relayR280 in its restored position and th sequence relay R250 operated, thetwo primary windings 2I5 and 2I6 are alternately energized under thecontrol of the pole changer contacts of the relay 225. The circuit forenergizing the primary winding 2I5 extends from ground by way of thearmature 226, the contacts 229, 252, 262, 212 and 283, the winding 2I5,the choke coil 22I and the conductor I6 to the negative terminal of thebattery I5. This circuit is obviously completed at the contacts 229 onlyduring the back stroke of the armature 226. The circuit for energizingthe primary winding 2 I 6 extends from ground by way of the armature226, the contacts 228, 253, 264, 214 and 285, the winding 2I6, thechokecoil 22I and the conductor I6 to the negative terminal of thebattery I5. This circuit is completed at the contacts 228 only duringthe forward stroke of the armature 226. The alternate energization ofthe primary windings 2I5 and 2 I 6 over the circuits just traced causesan induced alternating voltage to be developed in the secondary winding2 I4 having a frequency which corresponds to the frequency of vibrationof the relay 225. This voltage is impressed upon the ringing conductor292 over a path which includes the contacts 28I, 2'II, 26I and EM. Thecircuits over which the primary windings 2 l5 and 2 I6 are energizedunder the control of the other pulse generators 32, 33, etc., duringdifferent periods of each operating cycle of the sequence relays I9 arein all respects substantially similar to the circuits described aboveand will be fully apparent from the preceding explanation.

With the converting unit I8 included in the conversion channel, thecondenser 2I'I functions to minimize sparking and pitting of the polechanger contacts in the manner previously described. Also, rapidfluctuations in the voltage across the exchange bus conductors overwhich current is supplied from the battery I5 to the talking circuitsset up through the automatic switching equipment of the exchange, areprevented by the action of the high impedance choke coil 22!. Suchvoltage fluctuations tend to occur during these periods when the batteryI is not fully charged and the internal resistance thereof is low, andare occasioned by the heavy current surges which occur incident toclosing and opening of the circuits for energizing the primary windingsH5 and 216. The choke coil 22l tends to retard the current build-upduring each surge period with a resulting smoothing action on thevoltage of the battery l5. The retarding effect of the coil 22! is inpart compensated for by the action of the condenser 220 which is chargedduring each period when the respective circuits for energizing thevwindings 2l5 and 2N are both open, and alternately dis charges throughone of these windings and then the other as the energizing circuitstherefor are alternately completed. The radio frequency components ofthe voltage developed across the secondary winding of the transformer213 are suppressed through the action of the condenser 2l9. Further, theresistor 2H3 functions to damp the secondary winding 2M so that thepeaks which occur in the voltage produced across this winding at thebeginning and end of each voltage cycle are prevented from assumingabnormal values when only a light load is imposed on the apparatus.

As indicated above, the relay R295 is controlled in accordance with thecurrent traversing the screen electrodes 234d and 235d in parallel. Thepurpose of providing this particular circuit arrangement is to permitthe converter unit I! to be arranged for either class A, class B orclass C v operation, depending upon the particular output voltagerequirements. If an undistorted flat top output voltage wave is requiredthe unit may be operated class A in which case the two tubes 204 and 255are biased to operate on the straight portions of their grid-voltage,plate current curves. On the other hand, if distortion is permissibleand a large output is required, the tubes 204 and 295 may be biasedsubstantially to cut off or beyond for class B or class C operation. Inthe latter case, only a small current, if any, traverses the primarywindings 202 and 263 during periods when the pulse generators areinactive. Current of appreciable magnitude does, however, traverse thescreen electrodes 23M and 205d in the latter case. Accordingly, it ismore desirable from the standpoint of flexibility of operation, tocontrol the relay R295 in accord ance with the current traversing thescreen electrodes of the tubes rather than the current traversing theanodes of the tubes.

Referring now more particularly to the modified arrangement of theringing apparatus as shown in Fig. 3 of the drawings, this apparatuscomprises a transformer 300 having a secondary winding 30! which isarranged to be connected successively to the ringing conductors 392,333, 394, etc., under the control of a sequence switch 32!. Thetransformer 300 is also provided with a pair of primary windings 302 and303 which are arranged to be energized in opposite directions from adirect current source 35a over circuits which respectively include thespace current paths of the two thermionic tubes 304 and 365. These tubesare of the well-known pentode type and respectively include anodes 305aand 395a, cathodes 335C and 3050, control electrodes 304b and 335b,cathode heaters 30% and 395e, and screen electrodes 304d and 385d. Thecathodes of these two tubes are biased positively with respect to theirassociated control electrodes through the provision of a biasingresistor 306 which is connected to be traversed by the combined spacecurrents of the two tubes. An additional bias voltage derived from thevoltage dividing resistor 309 is impressed negatively upon the controlelectrodes 304D and 3052) through the resistors 301 and 308respectively,

For the purpose of varying the potentials of the control electrodes 30%and 30% alternately to in crease and decrease the current traversing theprimary windings 302 and 303 a plurality of electronic oscillators 3la,32a, 33a, etc., having different output frequencies are provided. Theseoscillators, which are of the well-known tuned-plate, tickler feed-backtype, are identical in wiring arrangement but have different circuitconstants in order to produce the required voltage of differentfrequencies. More specifically, the oscillator 33a comprises athermionic tube 3M of the wellknown pentode type. This tube is providedwith a tuned output circuit which includes the winding 3!! of thetransformer 3|5 and the portion 345a of a voltage dividing resistor 345.This circuit is tuned to the desired operating frequency by a condenser320 of suitable capacitance which is connected in shunt with thetransformer winding 3". The tube 3|4 is also provided with an inputcircuit which is coupled between the control grid 3l4b and the cathode3I4c and includes the transformer winding 3l8, a resistor 3|9, and theportion 3 I 0a of a voltage dividing resistor 3| 0. It will beunderstood that the voltage drop across the resistor section 3l0arepresents the biasing potential which is negatively impressed upon thecathode 3M0. The action of the resistor 3!!) in the oscillator inputcircuit serves to limit the grid current which may flow between thecontrol grid 3l4b and the cathode 3| 4c of the tube. The transformer 3l5also includes a tertiary winding 316 having a tapped center point whichis connected to the junction between the two resistors 30'! and 308. Theupper and lower terminals of the Winding 3|6 are connected tocorresponding contacts of the two contacts 323 and 324 in the sequenceswitch 32!, so that when this switch occupies a particular setting theoscillator 33a is connected to control the potentials of the controlelectrodes 30% and 3051). The center tap of the winding 3"? is alsomultipled to the center taps of the transformer secondary windings ineach of the other oscillators 3la, 32a, etc. These other oscillatorsalso derive cathode biasing potentials from the voltage dividingresistor 3 I i] and anode potentials from the voltage dividing resistor345 as indicated by the multiple connections extending thereto.

In order to control the sequence switch 321 so that the oscillators 31a,32a, 3311, etc., are successively connected to control the potentials'ofthe grids 30% and 305?), a timing relay R350 is provided which isshunted by a path which includes the condenser 329 and the resistor 330connected in series. The apparatus further comprises a start relayR360,a manually operable start switch 332, and a signal control relayR310. For the purpose of controlling the latter relay a signal controlnetwork is provided which includes the three electrode thermionic tube343 having its output electrodes 340a-and 3400 connected in circuit withthe relay R310 and its input electrodes 340b and 3400 coupled to thesecondary winding 3M of the transformer 300. The coupling networkbetween the transformer 300 and the tube 340 comprises ,a pair of seriesconnected resistors 34! and 342 of like resistance values which arebridged across the secondary winding 30'! a direct current blockingcondenser 343 and a biasing path which includes the resistor 344 and thesection 309a of the voltage dividing resistor 309'. The relay R310 isarranged directly to control a signal device 348 which may beconveniently located in the exchang wherein the illustrated ringingequipment is provided.

In considering the operation of the ringing ap-' paratus illustrated inFig. 3 of the drawings, it will be understood that this apparatus may bedirectly substituted for the apparatus shown in Fig. 2 of the drawings,to supply ringing voltages of the desired frequencies to the automaticswitching equipment illustrated in Fig. l of the drawings. In order tocondition the ringing apparatus for operation, the switch 332 isoperated to complete a circuit for energizing in parallel the cathodeheaters of the two tubes 304 and 305, the tubes respectively provided inthe various oscillators, and the signal control tube 340'. This circuitextends from the negative terminal of the exchange battery Ia by way ofthe current limiting resistor 33!, th contacts of the switch 332 and theparallel connected cathode heaters to the grounded terminal of thebattery l5a. When the cathode heaters of the respective oscillator tubesare energized to produce electron emission in these tubes, operation ofthe oscillators is initiated. In this regard it will be understood thatthe oscillator 33a, for example, is initially shock excited to developan oscillatory condition in the output circuit thereof. Due to thecoupled relationship between the two windings 3!! and 3l8 an oscillatingvoltage is impressed across the in put electrodes 3 I 4?) and 3 140 ofthe tube 3 l 4 which is of the correct phase and amplitude to sustainthe oscillatory condition of the tuned output circuit of the tube. Itwill also be undersood that the tuning of this output circuit determinesthe frequency of oscillation of the current traversing the winding 3 I7. Due to the coupled relationship between the two windings 3!! and 3l6the oscil'- lating current in the first of these two windings causes aninduced voltage to be developed in the winding sue. During succeedinghalf cycles of this voltage the upper terminal of the winding 316alternates between a value which is positive with respect to thepotential of the tapped center point of the winding and a value which isnegative with respect to this point. Each time the upper ter-' minal ofthe winding 3H5 becomes negative with respect to the tapped centerpoint, the lower terminal of the winding becomes positive with respectto this point. Conversely, each time the upper terminal of the winding 3It becomes posi tive with respect to the tapped center point of thisWinding, the lower terminal of the windingbecornes negative with respectto this point. Thus alternating control potentials are developed which,when impressed upon the control elec--' trodes 35419 and 335b, cause thecurrents traversing the primary windings 302 and 303 to be varied inopposite senses. In this regard it may be assumed that at the time theswitch 332 is operated to initiate the operation of the variousoscilla-' tors, the wipers 325, 326 and 32! of the sequence switch 32!occupy a setting wherein they engage their respective associated thirdcontacts. With the two wipers 320 and 32'! in this position the upperand lower terminals of the tertiary wind ing Sit are connected to thecontrol electrodes 33% and 305b, respectively. The resulting variationsin potential impressed upon these control electrodes are such that thecurrents traversing the primary windings 302 and 303 are varied inopposite senses to produce an induced alternating voltage in thesecondary windings 30!. In this regard it will be understood from theabove explanation that When the control electrode 304!) is made morepositive with respect to its associated cathode 3040 the controlelectrode 3051) is made correspondingly more negative with respect toits associated cathode 3050. Accordingly an increase in the currenttraversing the primary winding 302 is accompanied by a decrease in thecurrent traversing the Winding 303. The re+ verse action occurs when thepotentials of the control electrodes 304i; and 3051; are varied in theopposite sense. The rate at which these current changes occurcorresponds tothe oscillating frequency of the oscillator 33a, so thatan induced voltage of corresponding frequency is developed in thewinding 30!.

Normally the control electrode 3401; of the tube 330 isnegatively biasedwith respect to the oathode 340:: to apoint approaching anode currentcut-01f by the voltage drop across the section 309a of the resistor 309.In operation; this tube acts as a plate detector. Thus, when analternating voltage is developed across the secondary winding 30i of thetransformer 300, the major portion of the voltage wave is positivelyapplied to the control electrode 340!) through the condenser 343,whereby the negative bias on this grid is substantially reduced. Thedecrease in the average negative potentialon the control electrode 3401)is accompanied by a corresponding increase in the average currenttraversing the space current path of the tube 340. As a result thewinding of the relay R310 is sufficiently energized in a circuitincluding this path and the section 345a of the resistor 345 to operate.In operating, this relay opens its contacts 31! further to interrupt theincomplete circuit for energizing the signal device 348.

With the ringing apparatus in the condition described in the immediatelypreceding para graphs, an obvious circuit is completed for energiz ingthe start relay R360 from the exchange battery 65a when ground potentialis impressed upon the start conductors 335. In operating, this relaycloses its contacts 362 to prepare the operating circuit for the signaldevice 348. At its contacts 363, the relay R360 prepares a path overwhich the secondary winding 300 may succes'sively be connected to theringing conducf tors 332, 393, 394, etc., through the wipers" 325 of thesequence switch 32!. At its contacts 36!, the relay R363 completes theoperating circuit for the relay R350, this circuit extending from groundby way of the contacts 352, the winding of R350, and the contacts 36! tothe negative terminal of the battery l5a. Since the winding of the relayR350 is shunted by. theseries connected condenser 329 and resistor 330it will be understood that the operation of this relay is delayed untilthe condenser 329 is substantially fully charged. At the end of thisdelay interval the relay R353 opens its contacts 352 to interrupt itsoperating circuit and the circuit over which charging current isdelivered to the condenser 323. With these contacts open, the condenser329 is permitted to discharge through the resistor 33!] and the windingof the relay R350. At the end of the interval required to discharge thecondenser 329, the relay R350 releases and re'closes its contacts 352 torecomplete its oper-' ating circuit and the circuit for charging thecondenser 329. It will be understood from the above explanation that therelay 123% continues to operate and restore at a relatively slow rate solong as the start relay R360 is maintained in its operated position. Therate at which this relay alternately operates and restores is determinedby the capacitance value of the condenser 323, theresistance value ofthe resistor 33D and the impedance of the winding ofthe relay R3553.Each time the relay R350 operates it also closes its contacts dbl tocomplete a circuit including the contacts 36i for energizing the magnet323 from the exchange battery iiia; The circuit is obviously opened atthe contacts 35! each time the relay R35i3 restores. Each time themagnet 328 is thus energized it functions to advance the wipers 325, 326and 321 one step, whereby the different oscillators 3|a, 32a, 33a, etc.,are successively connected to control the space current paths of the'twotubes 384 and 305. Thus it will be noted that when these wipersengage their respective associated first contacts the oscillator am isconnected to control the two tubes 3% and 305 and hence the outputfrequency of the voltage developed across the secondary winding 30|.'Similarly when these wipers are operated to engage their respectiveassociated second contacts the oscillator 32a is connected to controlthe two tubes 3&4 and 3t5, whereby an output vpltage of differentfrequency is developed across the'secondary winding am. The sequenceswitch 32I also functions successively to associate the differentringing conductors 392, 393, 394, etc.,

with the secondary winding 36!. Thus when the wiper 325' is operated toengage its associated first contact, the conductor 39 i is connectedthrough the' contacts 363 to the high potential terminal ofthe winding30!. Similarly when the wiper 32'5.i's operated to engage its associatedsecond contact, the conductor 393 is connected to the high potentialterminal of the secondary winding 3M. It will be understood, therefore,that as the different oscillators are connected to control the frequencyof the voltage developed across the winding 30!, corresponding ones ofthe ringing conductors are connected to the high potential terminal ofthis winding.

As indicated above the relay R3?!) is normally maintained in itsoperated position so long as the frequency determin ng oscillators arefunctioning to cause the production of an alternating voltage across thesecondary winding 39!. If, however, one of these oscillators should failor one of the two tubes 30d and 365 should become defective, the outputvoltage across the secondary winding Sill will fail. When this conditionarises, the ne ative bias is restored to the control electrode of thetube 340?) with a resulting decrease in the space current through thistube and the winding of the relay R370. This relay accordingly releasesand closes its contacts 31! to complete a circuit including the contacts362 for'energizing the signal device 3 :8 from the exchange battery l5a.The operation of the signal device 348 may be utilized to inform theexchange attendant that the ringing apparatus is defective.Alternatively the signaling circuit just described may be utilized tocontrol automatic transfer means for cutting in a second set of ringingapparatus when the illustrated apparatus becomes defective to cause therelease of the relay R316. It will be noted that this signaling circuitis only completed through the contacts 362 when the start relay R350 isoperated. The

start relay in releasing also opens its contacts 36! to interrupt therespective operating circuits for the magnet 328 and the relay R'Jeiland opens its contacts 353 to interrupt the path over which thesecondary winding 384 may successively be connected to the ringingconductors esz, 333, 394, etc.

While there have been described what are at present considered to be thepreferred embodiments of the invention it will be understood thatvarious modifications may be made therein and it is contemplated tocover in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:

1. In a telephone system which includes lines and switching means forsetting up connectionsbetween said lines and for signaling over saidlines, ringing apparatus controlled by said switching means andcomprising, in combination, a transformer including a pair of primarywindings andia secondary winding adapted to be connected by saidswitching means to impress the voltage developed thereacross upon anyone of said lines, thermionic means including a pair of space currentpaths, a source of direct current, circuits commonly including saidsource and respectively including different ones of said paths anddifferent ones of said primary windings, and means for controlling saidpaths to produce alternate and periodic changes in the currenttraversing said circuits, whereby analternating voltage is developedacross said secondary winding.

2. In a telephone system which includes lines and switching means forsetting up connections between said lines and for selectively signalingover any one of said lines, ringing apparatus controlled by saidswitching means and comprising, in-combination, a transformer includinga pair of primary windings and a secondary winding,-thermionic meansincluding a pair of space cu-rrentpaths; a source of direct current,circuits commonly including said current source andrespectivelydncluding different ones of said paths and different ones ofsaid primary windings,

means for controlling said paths to produce alternate and periodicchanges in the current traversing said circuits, whereby an alternatingvoltage is developed-across said secondary winding, cyclically operatingmeans for periodically varying the periodicity ofthe changes in thecurrent traversing said circuits, whereby alternating voltages ofdifferent frequencies are developed across saidsecondary winding duringdifferent periods of'each operating cycle of said last-named means, andcircuits selectively controlled by said switching means for impressingthe voltage developed across said secondary winding on a selected oneofsaid lines only during one of said periods of each operating cycle ofsaid cyclically operating means.

3. In 'a telephone system which includes lines and switching means forsetting up connections between said lines and for selectiveiy signalingover any one of said lines, ringing apparatus controlled by-saidswitching means and comprising, in combination, a transformer includinga pair of primary windings and a secondary winding, thermionicmeansincluding a pair of space current paths and control electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including different ones'of saidpaths and different ones of said primary windings, means normallybiasing said electrodes to substantially the same potential, a pluralityof generators having different output periodicities and each operativeto vary the potentials of said electrodes to produce an alternatingvoltage across said secondary winding, a plurality of ringing currentconductors individually corresponding to said generators, means forsuccessively connecting said generators to control the potentials ofsaid electrodes and for simultaneously and successively connecting saidsecondary winding to corresponding ones of said conductors, and meansincluded in said switching means for selectively connecting saidconductors to a selected one of said lines.

4. In a telephone system which includes lines, switching means forsetting up connections be tween said lines, and frequency selectingswitches .for selectively connecting any one of a plurality of ringingcurrent conductors to a selected line. ringing apparatus controlled bysaid switching means and comprising, in combination, a transformerincluding a pair of primary windings and a secondary winding, thermionicmeans including a pair of space current paths and control electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including different ones of saidpaths and different ones of said primary windings, a plurality ofgenerators individually corresponding to said ringing current conductorsand having different output periodicities, each of said generators beingoperative to vary the potentials of said electrodes to produce analternating voltage across said secondary winding, and means forsuccessively connecting said generators to control the potentials ofsaid electrodes and for simultaneously and successively connectingcorresponding ones of said conductors to said secondary winding.

5. Ringing apparatus comprising a transformer which includes a secondarywinding and a primary winding having two sections, thermionic meanshaving a pair of space current paths, means for energizing the twosections of said primary Winding over different ones of said paths, andmeans for controlling said paths alternately to vary the energization ofsaid two winding sections in the same sense, whereby an alternatingvoltage is developed across said secondary Winding.

6. A converter comprising, in combination, a transformer including asecondary winding and a pair of primary windings, thermionic meanshaving a pair of space current paths, a source of direct current,circuits commonly including said source and respectively includingdifferent ones of said paths and different ones of said primarywindings, means for controlling said paths to produce alternate andperiodic changes in the current traversing said circuits, whereby analternating voltage is developed across said'secondary winding, andmeans for varying the periodicity of the changes in the currenttraversing said circuits, thereby to change the frequency of the voltagedeveloped across said secondary winding. I

7. A converter comprising, in combination, a transformer including asecondary winding and a pair of primary windings, thermionic meanshaving a pair of space current paths and including control electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including different ones of saidpaths and different ones of said primary windings, means for alternatelyand periodicall varying the potentials on said control electrodes toproduce an alternating voltage across said secondary winding, and meansfor intermittently varying the periodicity at which the potentialsimpressed on said control electrodes are varied.

8. A converter comprising, in combination, a transformer including asecondary winding and a pair of primary windings, thermionic meanshaving a pair of space current paths and including control electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including different ones of saidpaths and different ones of said primary windings, means normallybiasing said control electrodes to substantially the same potential, aplurality of pulse generators operative to produce pulses of differentperiodicities and each operative alternately to vary the potentials ofsaid control electrodes to produce an alternating voltage across saidsecondary Winding, and means for successively connecting said generatorsto vary the potentials of said control electrodes.

9. A converter comprising, in combination, a transformer including asecondary winding and a pair of primary windings, thermionic meanshaving a pair of space current paths and including contro1 electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including different ones of saidpaths and different ones of said primary windings, means normallybiasing said control electrodes to substantially the same potential, aplurality of thermionic oscillators operative to produce alternatingvoltages of different frequencies, circuits over which each of saidoscillators may be connected to vary the potentials on said controlelectrodes so that an alternating voltage is developed across saidsecondary Winding, and means for controlling said last-named circuits sothat said oscillators are successively rendered operative to control thepotentials of said control electrodes.

10. A converter comprising in combination, a transformer including asecondary winding and a pair of primary windings, thermionic meanshaving a pair of space current paths and including control electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including different ones of saidpaths and different ones of said primary windings, means normallybiasing said control electrodes to substantially the same potential, aplurality of generators having different output periodicities and eachoperative to vary the potentials of said electrodes to produce analternating voltage across said secondary Winding, a plurality of outputconductors .individually corresponding to said generators, and means forsuccessively connecting said generators to control the potentials ofsaid electrodes and for simultaneously and successively connecting saidsecondary winding to corresponding ones of said conductors.

11. A converter comprising in combination, a transformer including asecondary winding and a pair of primary windings, thermionic meanshaving a pair of space current paths and including control electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including different ones of saidpaths and different ones of said primary windings, means normallybiasing said control electrodes to substantially the same potential, aplurality of thermionic oscil lators operative to producealternatingvoltages of difierent frequencies and each operative to varythe potentials of said control electrodes to produce an alternatingvoltage across said sec ondary winding, a plurality of output conductorsindividually corresponding to said oscillators, and means forsuccessively connecting said oscillators to control the potentials ofsaid electrodes and for simultaneously and successively connecting saidsecondary Winding to corresponding ones of said conductors.

'12. A converter comprising, in combination,'a transformer including asecondary winding and a pair of primary windings, thermionic meansincluding a pair of space current paths and control electrodesindividual to said paths, a source of direct current, circuits commonlyincluding said source and respectively including diiierent ones of saidpaths and different ones of said primary windings, a plurality ofgenerators operative to produce voltages of different periodicities andeach operative alternately to change the potentials of said controlelectrodes in the same sense, whereby an alternating voltage isdeveloped across said secondary Winding, and means for successivelyconnecting said electrodes to be controlled in accordance with thevoltages of different ones of said generators.

13. Ringing apparatus comprising a ringing current conductor, a sourceof direct current, thermionic means having a pair of space currentpaths, a first conversion channel including said paths and operative toconvert the voltage of said source into alternating voltage and toimpress the alternating voltage on said conductor, a second conversionchannel for developing an alternating voltage independently of saidthermionic means and for impressing the alternating voltage on saidconductor, and means for selectively rendering said channels active.

14. Ringing apparatus comprising a ringing current conductor, a sourceof direct current, thermionic means having a pair of space currentpaths, a first conversion channel including said paths and operative toconvert the voltage of said source into alternating voltage and toimpress the alternating voltage on said conductor, a second normallyinactive conversion channel for developing alternating voltageindependently of said space current paths and for impressing thealternating voltage on said conductor, and means controlled inaccordance with the operative or inoperative condition of said firstchannel for rendering said second channel active.

15. Ringing apparatus comprising a ringing current conductor, a sourceof direct current, thermionic means having a pair of space current pathsand screen electrodes individual to said paths, a first conversionchannel including said paths and operative to convert the voltage ofsaid source into alternating voltage and to impress the alternatingvoltage on said conductor, a second normally inactive conversion channelfor developing alternating voltage independently of said space currentpaths and for impressing the alternating voltage on said conductor, adevice controlled in accordance with the current traversing said pathsby way of said screen electrodes, and means controlled by said devicefor rendering said second channel active when said first channel isrendered inoperative.

l6. Ringing apparatus comprising a ringing current conductor, a sourceof direct current,

thermionic means having a pair of space current paths, a firstconversion channel including said paths and operative to convert thevoltage of said source into alternating voltage and to impress thealternating voltage on said conductor, a second conversion channel fordeveloping alternating voltage independently of said space current pathsand for impressing the alternating voltage on said conductor, means forselectively rendering said channels active, a plurality of generatorscommon to said channels and having different output periodicities, andmeans for successively connecting different ones oi said generators tocontrol the frequency of the alternating voltage impressed on saidconductor through the one of said channels which is active.

17. A converter compri ng, in combination, a pair of transformers eachincluding a secondary winding and a pair of primary windings, thermionicmeans having a pair of space current paths, means including said spacecurrent paths for energizing the primary windings of one of saidtransformers, circuits for energizing the primary windings of the otherof said transformers, a pulse generator, operative to control said pathsso that the current traversing said path i alternately increased and tocontrol said circuits so that the primary windings of said othertransformer are alternately energized, and means for selectivelyrendering said generator operative to control said paths or saidcircuits.

18. A converter comprising, in combination, a pair of transformers eachincluding a secondary winding and a pair of primary windings, thermionicmeans having a pair of space current paths and screen electrodesindividual to said paths, means including said space current paths forenergizing the primary windings of one of said transformers, circuitsfor energizing the primary windings of the other of said transformers, apulse generator normally operative to control said paths o that thecurrents traversing the primary windings of said one transformer arealternately increased, said generator also being adapted to control saidcircuits so that the primary Windin-gs of said other transformer arealternately energized, and means controlled in accordance with thecurrent traversing said paths by way of said screen electrodes forrendering said generator inoperative to control said paths and forrendering said generator operative to control said circuits.

19. In combination With a ringing converter which comprises an outputtransformer having a secondary winding, an electron discharge devicecomprising a space current path and a control electrode for controllingthe current traversing said path, an output circuit including said spacecurrent path, means normally biasing said control electrode to apredetermined potential, circuit means for altering the potential ofsaid electrode in response to the energization of said secondarywinding, a signaling circuit, and means controlled in accordance withthe current traversing said output circuit for controlling saidsignaling circuit.

20. In combination with a ringing converter which comprises atransformer having a secondary winding, an electron discharge devicehaving input and output electrodes, an output circuit coupled to saidoutput electrodes, means normally biasing said inputelectrodes to apredetermined difference potential, an input circuit coupling said inputelectrodes to said Winding to change the difference potential of saidinput electrodes when said winding is energized, thereby to change thecurrent traversing said output circuit, a signaling circuit, and meanscontrolled in accordance with the current traversing said output circuitfor controlling said signaling circuit.

21. In a telephone system which includes lines and switching means forsetting up connections between said lines and for signaling over saidlines, ringing apparatus comprising, in combination, a ringing circuitover which current may be transmitted to a selected one of said lines, adirect current source operative to deliver energy to said circuit, anelectron discharge tube provided with a space current path, meansconnecting said space current path in circuit with said source, andmeans for varying the impedance of said space current path so that aperiodically varyin undulating current is transmitted to said circuit.

22. In a telephone system which includes lines and switching means forsetting up connections between said lines and for signaling over saidlines, ringing apparatus comprising, in combination, a ringing circuitover which current may be transmitted to a selected one of said lines, adirect current source operative to deliver energy to said circuit, anelectron discharge tube provided with a space current path, meansconnecting said space current path in circuit with said source, andfrequency determining means for varying the impedance of said spacecurrent path at a predetermined frequency so that a periodically varyingundulating current of said predetermined frequency is transmitted tosaid circuit.

23. In a telephone system which includes lines and switching means forsetting up connections between said lines and for signaling over saidlines, ringing apparatus comprising, in combination, a ringing circuitover which current may be transmitted to a selected one of said lines, asource of direct current, thermionic means operative to convert thevoltage of said source into alternating voltage and to impress thealternating voltage on said circuit, a plurality of frequencydetermining devices each operative to control the frequency of thealternating voltage impressed on said circuit by said thermionic meansand respectively operative to establish different frequencies of saidalternating voltage, and means for connecting different ones of saiddevices to control said thermionic means.

24. In a telephone system which includes lines and switching means forsetting up connections between said lines and for signaling over saidlines, ringing apparatus comprising, in combination, a ringing circuitover which current may be transmitted to a selected one of said lines, asource of direct current, thermionic means operative to convert thevoltage of said source into alternating voltage and to impress thealternating voltage on said circuit, a plurality of frequencydetermining devices each operative to control the frequency of thealternating voltage impressed on said circuit by said thermionic meansand respectively operative to establish different frequencies of saidalternating voltage, and cyclically operating means for sequentiallyconnecting said devices to control said thermionic means.

ROSWELL H. HERRICK.

